The techniques for drilling oil wells have considerably advanced during the last ten years or so and have therefore resulted in a new development of drilling tools used for cutting rocks.
The drilling tools used mostly in this field are bits, namely those of roller bit type which crush, chip fracture and scarenge the rock when the drill string is being rotated. However, these bits have relatively a short lifetime of from 15 to 20 hours and it is useful to check their wear so as to provide for their replacement. Problems of wear also exist in the case of P.D.C. (Polycrystalline Diamond Compact) bits, but the problem here is less critical.
A method is already known via the U.S. Pat. No. 4,627,276 on how to determine the mean values of the penetration rate of a drilling tool of the rotary speed and weight on bit so as to deduce drilling efficiency and the shearing strength of the tock where drilling is carried out.
The patent EP-A 0,168,996 shows for its part a device for detecting events during drilling as more particularly failure on drilling tools.
However, none of these documents offers a method for determining the progressive wear of cutting means, such as in particular the teeth of a roller bit. The method of the present invention, simple and convenient to operate, uses the measurement of two basic parameters, namely the weight on the tool W and the torque on the bit T, and the degree of wear can be obtained at any moment by the operator whenever he so wishes.